We propose to study the surface topography of lymphocytes by scanning electron microscopy and, in particular, to differentiate B and T lymphocytes and various possible subpopulations or subtypes. The mapping of immunoglobulin and histocompatibility antigens will be established using monovalent antibodies suitably labeled and examined by freeze etch and scanning electron microscopy. The mechanisms of capping of Ig in particular as it relates to microtubular and/or microfilament control will be assessed. The main objective is to establish the randomness or nonrandomness of localization of these entities in the membrane and the factors that impose organization on them. Finally, we wish to map the surface of macrophages in regard to antigens, or Fc/C3 sites, and the control of their behavior during endocytosis. BIBLIOGRAPHIC REFERENCES: Abbas, A.K., Dorf, M.E., Karnovsky, M.J., and Unanue, E.R.: The distribution of Ia antigens on the surfaces of lymphocytes. J. Immunol., 116: 371, 1976. Abbas, A.K., Unanue, E.R., and Karnovsky, M.J. Current techniques for the detection of surface immunoglobulin on lymphocytes. Techniques of Biochemical and Biophysical Morphology, Vol. 3, 1976, in press.